Dry silver photosensitive compositions comprise an intimate mixture of a light sensitive silver halide and another silver compound such as a silver salt of an organic acid, e.g. silver behanate or silver saccharine, which upon reduction gives a visible change and which is substantially light-insensitive. Such a mixture is usually prepared in suspension and the resulting dispersion spread as a layer on a suitable substrate. When dry, the layer is exposed to a light image and thereafter a reproduction of the image can be developed by heating the layer in the presence of a reducing agent such as hydroquinone or certain substituted phenols.
It is because the exposure and development of the layer occur without using water, that these materials are often referred to as dry silver light-sensitive materials. Such materials in which minor amounts of a photosensitive silver halide catalyst-progenitor are associated in catalytic proximity with major amounts of a heat sensitive oxidation-reduction image forming reaction mixture which reacts more rapidly in the presence of the catalyst resulting upon exposure of the silver halide are well known in the art. Examples of such materials are described in our British Pat. No. 1,110,046 and in U.S. Pat. Nos. 3,839,049 and 3,457,075.
We believe, that when the mixture is exposed to light, a latent image is formed in the silver halide. Thereafter, the silver compound can be reduced by heating with the reducing agent, this reduction being catalysed image-wise by the light exposed silver halide. By a suitable choice of temperature, the reduction of the silver compound can be catalysed in the light exposed areas to give a visible darkening while any slight reduction which occurs in the non-light exposed areas is insufficient to give a marked change. Of course, because the silver halide acts as a catalyst progenitor, only very small amounts of it can suffice, e.g. 0.1 to 10% by weight of the mixture. However, large amounts, e.g. up to 15 or even 20% may be desirable in some circumstances.
In order to improve the sharpness or definition of photographic images a dye known as an acutance dye is often incorporated into photosensitive compositions. To be effective the acutance dye will absorb at the wavelengths at which the photosensitive composition is sensitive. The longer the path length of the light in the layer of light sensitive composition the greater the attenuation. Therefore, scattered light is attenuated or absorbed to a larger extent that light which impinges directly on a light-sensitive crystal. As a result therefore, although the overall speed of the composition is reduced slightly, scattered light and other light rays which are liable to produce a blurred image are preferentially absorbed and so the overall definition and sharpness of images produced in the layer are increased.
An acutance dye for use in a dry silver composition is preferably heat labile, that is to say, it is destroyed by the heat development of the dry silver composition to one or more compounds which are substantially colourless.
It is therefore an object of this invention to provide dry silver compositions containing acutance dyes, which absorb light at at least some of those wavelengths to which the composition is sensitive and which are rendered colourless upon heat development of the dry silver composition.
It is also an object of the invention to provide novel compounds suitable for use as acutance dyes in dry silver systems and a method for their preparation.